A common requirement in mechanical systems is the ability to tighten a bolt with a known torque. A typical example is the tightening of "head bolts" on an automobile engine, an event which occurs many times a day in auto repair facilities.
In the prior art, the torque is applied with a special wrench known as a "torque wrench". To apply the desired torque, the operator must watch the movement of a pointer against a calibrated faceplate on the wrench during tightening. A difficulty with this device is that direct observation of the wrench faceplate is difficult or impossible in many situations. A solution to this problem is achieved in the prior art by the use of a mechanically similar ratchet-like structure which sounds an audible "click" when a preset torque value is attained.
However, torque wrenches of this type are very expensive, relative to standard type torque wrenches.